Corrosion resistant coating for metal surfaces



Feb.,9, 1943. G. w. JERNs-rEDT 2,310,239

CORROSION RESISTANT COATINGS'FOR METAL SURFACES Filed oct'. 2s, 1941 ssheetssheet 1 INVENTOR 6M .ifea/ l Feb. 9, 1943. G. w. JERNsTEDT2,310,239 v CORROSION RESISTANT COATINGS FOR METAL SURFACES Filed Oct. 25, 1941 3 Sheets-Sheet-2 Feb 9 1943- G. w. JERNsTl-:DT 2,310,239

' CORROSION RESISTANT COATING'S FOR` METAL SUFACES Filed Oct. 25, `19413 vSheens-Shee't. 3

Patented Feb. 9, 1943 q `amazes CORROSION RESISTANT COATING FOR METALSURFACES George W. Jernstedt, Bloomfield, N. J., assignor toWestinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., acorporation of Pennsylvania Application October 25, 1941, Serial No.416,552

produced by simply subjecting the metal surface 8 Claims.

This invention relates to the art of producing corrosion resistantcoatings on the surfaces of ircn. zinc and other metals and alloys.

This invention relates ymore particularly to the production of phosphatecoatings upon the surfaces of metals. Such phosphate coatings are ofgreat economical importance in the preparation of the metal surfaces forthe reception of an organic finish.

The object -of this invention is to provide a preliminary treatment formetal surfaces to accelerate the formation of corrosion resistingcoatings thereon.

.Another object of this invention is to provide a composition capable ofactivating metal surfaces to improve the process of forming corrosionresisting coatings thereon. v

A further object of the invention is to provide a treatment for metalsurfaces to .produce protective phosphate coatings in a rapid andeconomical manner.

Other objects of the invention will, in part, be obvious and will, inpart, appear hereinafter.

For a fuller understanding `of the nature and objects of the invention,reference may lbe had to the following detailed description taken inconjunction with the accompanying drawings, in which: v

Figure 1 is a schematic view of one form of process for accomplishingthe invention;

Fig. 2 is a fragmentary sectional view of a metallic member carrying aprotective coating produced by the treatment described herein.

Fig. 3 is a photomicrograph at 300X of zinc plated steel with ordinaryphosphate treated surface;

Fig. 4 is a .photomicrograph at 300X of zinc plated steel treatedaccording to.the invention.

Fig. 5 is a photomicrograph at 300X of steel with ordinary phosphatesurface treatment; and

Fig. 6 is a photomicrograph at 300X of steel treated according to theinvention.

I'he treatment of metallic surfaces with solutions or compositionswhich, under appropriate conditions, will deposit or form upon themetallic .surface a protective phosphate coating is practicedextensively. These phosphate coatings protect the underlying metal fromcorrosion. Furthermore the phosphate coatings are desirable since theyconstitute excellent surfaces for the successful application of organic,finishes. The phosphate coatings occur as crystalline deposits towhich'organic finishes will bond and adhere more tenaciously than to thebare metal surface.

In the prior art, phosphate coatings have been to a solution containingphosphoric acid and phosphates. The time involved in treating the metalsurface in order to secure a satisfactory protective coating usually hasbeen reckoned in hours, but within recent years the time of treat. ment,has been reduced to a fraction of anzhour, for example, 15 minutes.

According to the invention, it has `c'een discovered that the timenecessary for the reaction .between the metal surface and a compositioncapable of developing the phosphate coating thereon depends to a greatextent upon the relative activity of the metal surface to thecomposition. When treated with an activating solution as will bedescribed herein, the bare metal surface has imparted to it improvedcharacteristics whereby it is activated sol that the normal or vmodifiedphosphate coatingv compositions will form a phosphate lm or coatingthereon with great rapidity, for example, in a minute or less.

Another advantage obtained by the activation of metal surfaces is thatmetal surfaces which previously did not form satisfactory phosphatecoatings, successfully acquire phosphate good corrosion inhibitingcoatings upon activation. As is well known, zinc does not form goodprotective phosphate coatings when coating compositions are applied, butwith activation zinc will acquire coatings capable of meeting extremecorrosion requirements.

When metal surfaces are activated by a pretreatment, as disclosed inthis invention, the surto that secured on unactivated surfaces.

It is well known to those skilledin the art that zinc, galvanized ironand electroplated Zinc cannot be treated with uniformly satisfactory andconsistent results with conventional phosphate solutions. Numerousattempts to modify the phosphate solutions in'order to produce anadequate protective phosphate coating on zinc have been unsuccessfulfrom a commercial standpoint.

, POSE.

-di'um phosphate will activate metal surfaces.

In contrast to the prior art dimculties with zinc, zinc surfaces afteractivation may be subjected to the phosphate coating solutions withphenomenally rapid production of crystalline deposits of highestquality.

In the practice of this invention, metal surfaces to be subsequentlysubjected to a treatment capable of producing crystalline phosphatecoatings are activated by pretreatment with an activating solution. Thepretreatment comprises the application of a solution of disodiumphosphate containing additions of small quantities of titanium as acompound. In the absence of titanium, the disodium phosphate hasnegligible activating effgit.

In preparing a satisfactory disodium phosphate and titaniumpretreatment, the following process has been found to be successful. Ihetitanium compound is added to ortho-disodium phosphate in watersolution. Preferably soluble titanium compounds are employed foradditions to the disodium phosphate. Titanium tetrachloride, titaniumtrichloride, titanium hydroxide, titanium nitride and titanium potassiumoxalate have been employed for this purpose with highly satisfactoryresults. Less soluble compounds of titanium such as titanium carbide.titanium dioxide and titanium potassium fluoride however, have beenadded to disodium phosphate and al1 have rendered the disodium phosphatecapable of activating metal surfaces. The solu. tion of disodiumphosphate with the titanium compound addition is preferably evaporatedto dryness at temperatures above 60 C.

It has been found that disodium ortho-phosphate is the only materialwhich is satisfactory in producing the desired activation of metalsurfaces. Dipotasslum phosphate and both monosodium phosphate andtrisodium phosphate, for example, were substantially useless for thispur- 'Ihe only compound, apart from disodium phosphate, of many testedthat exhibited some activating characteristics was sodium pyro phosphate(Na4P20r). Disodium phosphate has been found to be the most satisfactoryfor activating metal surfaces.

The dried disodium phosphate-titanium compound composition as preparedabove may contain from 0.005% up to 20% by weight of titanium radical.The dried composition is dissolved in water to produce a. solutionsuitable for use in activating treatment, having from 0.1% to 2% ofdisodium phosphate and generally from 0.005% to 0.05% of titanium ionsin solution. Titanium may be present in larger amounts with beneficialresults. However, it has been found that from 01% to a saturatedsolution of diso- A 1% aqueous solution of disodium phosphate with 0.01%titanium radical appears to be the optimum concentration. The pH at thisconcentray tion is from 8.0 to 8.5.

Referring to Fig. 1 of the drawings, there is shown a schematic processwhich may be followed in applying the activating pretreatment to metalsurfaces including the subsequent treatment in producing the' coatedmetal surface.

Metal whose surface has been cleaned thoroughly is introduced into tankI0. At I is an apparatus. either a tank in which the metal is to bedipped in the activating solution I2, or a booth in which the activatingsolution is sprayed upon the members being treated. The disodiumphosphate and titanium solution I2 need be applied for only 10 secondsto the metal surface to con- Percent Iron 0.03 to 0.30 Zinc phosphate0.04 Sodium nitrate 0.2 to l Phosphoric acid to produce a 20 pointsolution and comprising 1:8 of total phosphate content.

Remainder water.

In 30 seconds or less a highly effective phosphate coating will bedeposited on the zinc surfact. Longer applications of the phosphatesolution are also successful.

In the prior art, unactivated metal surfaces required treatment with aphosphate solution for 15 to 20 minutes. For example, in applyingphosphate coatings to automobile bodies, several thousand gallons ofsolution were sprayed upon the metal before an adequate amount ofphosphate coating was formed. In addition, the coatings were non-uniformand it was desirable to paint such coatings as soon as possible. Whenpre-activated, metal surfaces may be sprayed or dipped in the phosphatesolution 16, for a period of time of the order of one-half minute, toproduce and extremely uniform and lne phosphate coating.

The metal forms largequantities of gaseous bubbles when the solution isfirst applied to the activated surfaces. It is believed that the bubblesconsist mainly of hydrogen gas. After a. fraction of a minute, thebubbling ceases and the reaction is substantially complete. It isgenerally desirable to prolong the application of the solution to themetal surface for a short period of time beyond the point at which thegaseous bubbles cease to be given oil'.

The extremely rapid formation of a protective phosphate coating at thisstage of the process constitutes one of the unexpected results of theinvention. In particular, zinc surfaces acquire an almost velvety andclean-appearing surface of very fine crystalline nature.

In treating ferrous metals, for example-stampings of all types, asolution for use in receptacle i4 more suitable than the one given abovefor zinc is suggested:

Manganese 4phosphate -pounds-- 1% Phosphoric acid 83% .-do 5 Sodiumnitrate dn 1/2 Cupric nitrate ounce-- 1/4 Water to make one gallon.

Therefore, a non-uniform coating may result.

The oxidizing agent will immediately react with the bubbles of nascenthydrogen and remove them from the surface of the. metal. Otherloxidizing agents, such as sodium or potassium nitrate, are suitable forthis purpose.

Zinc phosphate may be introduced to replace a part of the manganesephosphate in the solution for treating ferrous metals` Generally, thepresence' o'f one or more of the group consisting of zinc, copper andmanganese greatly expedites the reaction of the ferrous metal with thephosphate solution.

Numerous other solutions capable of producing phosphate coatings uponmetal are known to the art and their reaction with activated metalsurfaces is equally feasible to produce a protective coating.

After an adequate crystalline phosphate coating has been produced uponthe metal surface, the metal surfaces are rinsed with water I8 containedin receptacle 20. For best results, the surfaces are subjected to asealing treatment at 22 with chromic acid solution 2l. The chromic acid24 consists of an Iaqueous solution of 71A, ounces of Cr(OH3) per 100gallons of water. The water rinse 20 is not necessary with the sealingtreatment of chromic acid. Only a few seconds application of the chromicacid solution is necessary to secure a sealing of the phosphate coatingto improve the corrosion resistance. Y

The metal, Aafter a sealing treatment at 22, may be passed through adrying oven 26 to completeiy dry the surface for subsequent operations,such as applying of an organic finish.

Alternatively, it has been discovered that the water rinse and dryingoven stcps in the process may be dispensed with if the chromic acidsolution 24 is heated-to a temperature of vabout 190 F., or even higher,suicient heat being imparted by the hot chromic acid solution to themetal so that after removal from the solution, the metal driesautomatically in air.

Microscopic examination of the coated metal surface reveals a structuresomewhat as shown in Fig. 2. The base metal has a 'distinct and separatelayer 32 which is of substantially uniform thickness, and appears to bekeyed to the metal 30. The coating 32 is extremely durable and willwithstand both chemical and mechanical abuse. remove the coating 32.Such coating is uniformly deposited over all the recesses and otherirregularities in the surface of metal-body 30.

Referring to Figs. 3 to 6, there are shown photomicrographs at a.magnification of 300X of sam'ples of material treated with prior artphosphate solutions alone, and treated as disclosed herein.

Fig. 3 shows a coarse, crystalline surface deposit of phosphates-on zincplated steel. This was produced by applying a phosphate-phosphoric acidsolution alone. The deposit is nonuniform withv large spaces betweencrystals. Such a surface deposit produces an unsatisfactory protectivecoating.

The coarse nature of the phosphate deposit in Fig. 3 requires theapplication of two or more coatings of organic finish to produce asmooth surface. The organic finish does not adhere any `too well to thelarge crystals and its life is relatively short.

For comparison, Fig. 4 shows a similar zinc plated steel member as inFig. 3, with a phosphate coating produced by a predip in a disodiumphosphate-titanium solution followed by an application of the samephosphate coating solution as Abrasion or scratching is necessary toempioyed alone for the Fig. 3 coating. The miiform and fine grainedphosphate deposit tn Fig. 4 is an outstanding feature. There are no openspaces in Fig. 4 as exist in Fig. 3.

The fine grained phosphate coating in Fig. 4 has beensatisfactorilycoated with a single coating4 or'organic finish to produce a smooth,commercial outer surface. Due to its better corrosion-proonng organicfinishes last longer when applied to the Fig. 4 surface. The bonding oi'the finish tothe `metal is enhanced by the fine grained phosphatedeposit. y

Fig. 5 is a photomicrograph at a. magnification of 300X of an ordirmrysteel member with the usual type of phosphate coating produced thereonby the application of a solution of phosphates and phosphoric acidalone. Not only is the protective phosphate coating coarse grained, butits protection is limited. lIt is customary to require that organicfinishes be applied to ferrous metals within a few hours after thephosphate The eectiveness of the predip in producing a o fine grained,highly protective phosphate is ev'ldent. The Fig. 6 phosphate coatingshowsthe same superiority over lthe prior art coatings as does the zincplated member subjected to the predip.

While occasionally in prior art practice protective coatings have beensecured whose appearance to the naked eye appears to correspond to thatproduced by the present process involving an activating pretreatment,corrosion tests have revealed a decided difference in the quality of thecoatings with better corrosion resistance produced by the invention inthis case. Ordinary zinc plating on a steel base will turn completelywhite after 12 hours in a steam chest at 140 F., due to the corrosion ofthe zinc. The same type of zinc plated member subjected to a standardphosphate `composition treatment, without an activating pretreatment,exhibits an improved resistance to corrosion in the steam chest: After1.2 hours a few White spots will be apparent on the surface of suchmetal. When zinc plated members similar to those tested above werevgiven an activating pretreatment, followed by a phosphate surfacetreatment as detailed in Fig. l, at least 14 days elapsed before anysample showed asign of corrosion in the steam chest operating at F.Generally the members are capable of withstanding a month in the steamchest before any extensive degree of corrosion is evidenced in activatedmaterial.

When the three types of material were additionally coated with lanorganic finish, an almost equivalent proportionality in the resistanceto corrosion was obtained. Samples to which an" activating pretreatmentwas applied to zinc prior to a phosphate treatment lasted over 25 timeses long at 140 F.'ln a steam chest before failure of the organic finishwas observed, as compared to the other types of material.

It will be seen by this process that not only has the processing timebeen cut down to a minimum, as compared to the vtime required in the lprior art to produce aphosphate coating, but

also coatings of a superior corrosionresistance are deposited or formedon the metal. Furthermore, certain metals which have been treated priorart in which two or more applications of organic iinish have previouslybeen necessary.

Since certain changes may be made in the above invention and dierentembodiments of the invention may be made Without departing from thescope thereof, it is intended that all matter contained in theabove-described disl closure or taken in connection with theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

I claim as my invention:

1. A composition which when in aqueous solution is capable of activatingferrous and zinc metal surfaces to provide for the application. ofprotective phosphate coatings which comprises disodium phosphate andmore than 0.005% by weight of titanium, the titanium being present as acompound.

2. A composition'which when in aqueous solution is capable of activatingferrous and zinc metal surfaces to provide for the application ofprotective phosphate coatings which comprises 80% and cver of disodiumphosphate and from 0.02% to by weight of titanium, the titanium beingpresent as a compound.

3. An aqueous solution for use in activating ferrous and zinc metalsurfaces comprising from 0.1% to 2.0% of disodium phosphate and 0.005%to 0.05% of titanium, the titanium being present as a compound.

4. An aqueous solution for activating ferrous and zinc metal surfaces toaccelerate the formation of corrosion resisting coatings thereoncomprising disodium phosphate and at least one t1- tanium compound.

5. In the process of treating ferrous and zinc metal surfaces prior toproducing thereon corrosion resisting .phosphate coatings, the stepcomprising applying to the metal surface an aqueous solution of disodiumphosphate and at least one titanium compound to activate the metalsurface.

6. In the process of treating ferrous and zinc metal surfaces prior toproducing thereon corrosion resisting phosphate coatings, the stepcomprising applying to the metal surface an aqueous solution containing0.1% to 2% disodium phosphate and from 0.005% to 0.05% titanium toactivate the metal surfaces, the titanium being present as a compoundwhich may be ionized.

7. 'Ihe process of treating ferrous and zinc metal surfaces to providecorrosion resisting coatings thereon which comprises, in combination,applying a solution of disodium phosphate and titanium ions to activatethe metal surface and subsequently applying a solution of phosphoricacid, phosphates, and an oxidizing agent to the activated metal surfaceto produce a protective phosphate coating thereon.

8. The process of treating ferrous and zinc metal surfaces to providecorrosion resisting coatings thereon which comprises, in combination,applying an aqueous solution having from 0.1% to 2.0% disodium phosphateand 0.005%

to 0.05% titanium, the titanium being present as a compound, to activatethe metal surface and applying a solution of phosphoric acid,phosphates, and an oxidizing agent to the activated metal surface toproduce a protective phosphate coating thereon.

GEORGE W. JERNS'I'EDT.

